Electrochemical energy storage is currently used in many different portable applications, such as wireless communications and portable computing, just to name a few, and will be essential for the realization of future fleets of electric and hybrid electric vehicles, which are now believed to be an essential part of the world's strategy for reducing our dependence on oil and minimizing the impact of gaseous emissions of CO and CO2 on climate change. In looking at possible materials that can be used for anodes in electrochemical energy conversion and storage systems, hydrogen and lithium are two of the lightest elements and have the highest specific capacities, usually given in units of Ah/kg. See FIG. 1 which is a plot showing rankings of conventional anode materials. Hydrogen is typically used to power fuel cells, while lithium is typically used in advanced rechargeable battery cells and batteries.
Most currently used energy storage systems use lithium-ion battery chemistry, with graphite anodes that intercalate lithium upon charging, mixed transition metal oxide cathodes that intercalate lithium during discharge, a micro-porous polyethylene electrode separator, and an electrolyte formed from a dielectric mixed-solvent composed of organic carbonates, with an appropriate dissolved high-mobility lithium salt. The movement of the lithium ions between the intercalation anodes and cathodes during charging and discharging is commonly known as the “rocking chair” mechanism.
Cells with liquid electrolytes are usually housed in cylindrical or prismatic metal cans, with stack pressure maintained by the walls of the can, while cells with polymer gel electrolytes are usually housed in soft-sided aluminum-laminate packages, with stack pressure achieved through thermal lamination of the electrodes and separators, thereby forming a monolithic structure.
The coating of active material on the copper anode foil includes graphite powder with a conductive carbon filler and a polyvinyldine fluoride (PVDF) binder, while the coating of active material on the aluminum cathode foil includes a transition metal oxide powder, also with a conductive carbon filler and a PVDF binder. Both natural and manmade graphite, such as mesocarbon microbeads (MCMB), have been used for the anodes, while LixCoO2, LixNiO2, LixMn2O4, mixed transition metal oxides with cobalt, nickel, and manganese, and iron-phosphates, among others, are common choices for the cathode.
Over the past decade, these systems have attained outstanding specific energy and energy density, exceptional cycle life and rate capabilities that enable them to now be considered for both vehicular and power tool applications, in addition to their early applications in wireless communications and portable computing. The best commercially available, polymer-gel lithium ion battery now has a specific energy of greater than 180 Wh/kg, an energy density of greater than 360 Wh/L, and a reasonably good rate capability, allowing discharge at C/2.
The specific energy is calculated by dividing the total energy stored in the lithium-ion cell by the total mass of material involved in the cell construction, while the energy density is calculated by dividing the total energy stored in the lithium-ion (or other) cell by the total volume of material involved in the cell construction. The cell mass includes those materials used for fabrication of the graphite intercalation anodes, transition metal oxide intercalation cathodes, separators, and cell packaging.
For example, both liquid prismatic and polymer gel cells may be incorporated into large high-capacity power packs and used to power large energy users, such as large mobile lasers. Such high capacity systems have state-of-the-art computerized charge and discharge control which includes graphical user interfaces, sensing for monitoring the health of individual cells, and charge balancing networks. Such lithium ion batteries, which rely on the rocking chair mechanism, are generally believed to be safer than those where lithium exists in the reduced metallic state.
The use of flammable completely liquid-phase and polymer-gel electrolytes with a substantial fraction of liquid electrolyte, coupled with a high energy density, a relatively delicate (about 20 micron thick) polymeric separator, and the possibility of lithium plating and dendrite formation due to non-uniform stack pressure and electrode misalignment, as well as sharp edges on electrode foils and foreign objects and debris trapped between the electrodes, have led to catastrophic internal shorts, followed by the onset of thermal runaway, electrolyte combustion, and other serious safety problems associated with these energy storage systems. One example of the type of unanticipated event with a lithium ion battery is evidenced by the rash of exploding laptop batteries experienced several years ago. The possibility of such an event occurring on commercial airliners, where many passengers carry laptop computers and cell phones with such batteries, is especially disconcerting. These events have occurred on much larger scales, and have caused industry-wide concern in the continued use of this important technology.
In conventional lithium-ion cells, thermal runaway can lead to disproportionation of the transition metal oxide cathode, thereby liberating sufficient oxygen inside of the closed cell volume to support oxidation of the organic carbonate solvents used in the liquid or polymer-gel electrolytes. Therefore, it would be very beneficial to develop new battery materials that enhance the performance of rechargeable solid-state lithium-ion batteries, and that will provide high specific energy, high volumetric energy density, and high rate capability at high and/or low temperatures, e.g., about 0° C., with substantially improved safety and reliability.